In recent years, mobile communications fields represented by car phones and cell phones have seen some drastic growth. And then, high-frequency bands referred to as the so-called quasi-microwaves of the order of several hundred MHz to a few GHz are used for those mobile communications. In electronic devices used with mobile communications equipment, like resonators, filters and capacitors, too, high-frequency characteristics are thus of importance.
Important factors for the recent widespread use of mobile communications are, to say nothing of more sophisticated services, size and cost reductions of communications equipment. This in turn leads to the need of reducing the size and cost of associated high-frequency devices. For instance, to achieve size reductions in terms of resonator material, there are dielectric porcelain compositions required, which have a high specific dielectric constant and limited dielectric losses at the frequency used, and which are reduced in changes in the temperature characteristics of resonance frequency.
For materials for high-frequency devices that meet such demands, there have been BaO-rare earth oxide-TiO2 base dielectric porcelain compositions known so far in the art.
Now, to downsize high-frequency devices, there are surface mount devices (SMDs for short) going mainstream, which each have conductors such as electrodes and wirings. In what follows, a conductor such as an electrode or wiring built in a high-frequency device will be called an “internal conductor”.
To form conductors such as electrodes or wirings in the device, it is required that a dielectric porcelain composition be co-fired with them. Of a BaO-rare earth oxide-TiO2 base dielectric porcelain composition, however, the firing temperature is relatively high, say, 1,300 to 1,400° C., and materials for conductors such as electrodes or wirings, used in combination with it, have been limited to noble metals such as palladium (Pd) and platinum (Pt) that can stand up to high temperatures.
Because those noble metals cost much, however, it is desired for the purpose of making devices less expensive that low-resistance yet less expensive conductors such as Ag or Cu can be used as the internal conductors.
To this end, there has been a technique proposed, in which subordinate components such as B2O3 are added to a material composed mainly of a BaO-rare earth oxide-TiO2 system. With that technique, a dielectric porcelain composition can be fired at a temperature lower than the melting point of a conductor such as Ag or Cu, so that the conductor such as Ag or Cu can be co-fired in the form of an internal conductor (see, for instance, JP-A's 2001-31468 and 6-40767).
To achieve further size reductions of devices, on the other hand, there has been a multilayer type device of high performance proposed, in which a dielectric porcelain composition having a high specific dielectric constant is joined to a dielectric porcelain composition having a low dielectric constant, thereby forming a plurality of high-frequency devices as one integral piece (see, for instance, JP-A 9-139320).
However, if, in forming such a multilayer type device, the dielectric porcelain composition having a high specific dielectric constant differs in properties from that having a low dielectric constant, defects will occur at a interface of both upon firing, because the shrinkage behaviors and linear thermal coefficients of both do not match.
In view of such considerations, it is desired that for multilayer type device formation, the dielectric porcelain composition having a high specific dielectric constant and the dielectric porcelain composition having a low specific dielectric constant be basically made up of the same or a similar material having much the same physical properties.
However, the BaO-rare earth oxide-TiO2 base dielectric porcelain composition well fit for the material of a small high-frequency device has a much higher specific dielectric constant, as set forth in the aforesaid patent publication 1, and even with simple addition of subordinate components, it would be difficult to produce a BaO-rare earth oxide-TiO2 base dielectric porcelain composition that has a low specific dielectric constant as demanded for a composite arrangement (multilayer type device).
To fabricate a multilayer type device having limited power losses for high-frequency purposes, there are also growing expectations of a BaO-rare earth oxide-TiO2 base dielectric porcelain composition that is much more improved in terms of dielectric losses than could be possible with the prior art.